Example failed task: https://chrome-swarming.appspot.com/task?id=3e7556dca5c95110

Looking at 
chromeos-test@pprabhu-skylab-drone-1:/usr/local/autotest/results/swarming-3e7556dca5c95111/chromeos6-row2-rack8-host10$ less debug/autoserv.DEBUG

...

07/02 12:43:31.930 ERROR|          autoserv:0813| Uncaught Exception, exit_code = 1.
Traceback (most recent call last):
  File "/usr/local/autotest/server/autoserv", line 805, in main
    use_ssp)
  File "/usr/local/autotest/server/autoserv", line 525, in run_autoserv
    host_attributes=parser.options.host_attributes,
  File "/usr/local/autotest/server/server_job.py", line 132, in get_machine_dicts
    machine)
  File "/usr/local/autotest/server/server_job.py", line 1491, in _create_file_backed_host_info_store
    % backing_file_path
AutoservError: Requested FileStore but no backing file at /usr/local/autotest/results/swarming-3e7556dca5c95111/chromeos6-row2-rack8-host10/chromeos6-row2-rack8-host10.store